Vacuum tube circuit



March 19, 1935. A. v. WURMSER v VACUUM TUBE CIRCUIT Filed Aug. 18, 1953 2 Sheets-Sheet 1 A TTENUATION EQUAL/ZEN IN [/5 N TOR A. I! WURMSER BVJ A TTOR/VE Y March 19, 1935.

A. V. WURMSER VACUUM TUBE C IRCUIT Filed Aug. l8, 1953 2 Sheets-Sheet 2 lNl EN 7-0;? A. 1/. WURMSER ATTORNEY Patented Mar. 19, 1935 I r 1 994 498 UNITE STATES PATENT OFFICE VACUUM TUBE CIRCUIT Alphons Wurmser, Teaneck, N. 1., assignor to Bell Telephone Laboratories, Incorporated,

New Yorh'N. Y.,a corporation of New York Application lnugust 18, 1933, Serial No. 685,696

' ,7 Claims. (or. 178- 44) This invention relates to vacuum tube circuit back, and herein, as in that application, the feedas for example vacuum tube amplifiers. back is described as positive feedback or negative Objects of the invention are to control phase feedback according as the'absolute value of and amplitude of waves in such circuits. t 1 5 It is also an object ofthe invention to'so con- T27 trol phase and amplitude of waves as to reduce I H i singing tendency in vacuum tube amplifiersthat is greater or less than unity. feed back modulation 'or distortion components In one Specific aspect the invention s embodie for reducing distortion, or feed back fundamental in Vacuum tube amplifiers 0f the general yp in components for increasing stability, 01 both. which Waves including those of the range of Such amplifiers are disclosed, for example in a transmitted frequencies, ar S fed back from the c-opending application 0f 11 s Black, g output to the input as to reduce the gain of the 411,223, filed Decemb 3 1929 r Wave transamplifier below the value that it would have withlation systems, and i iti h p t t 317,995 out feedback in order to reduce unwanted modu- Certain terms nd symbols used herein have lation or nonlinear efiects and render the gain 15;

the following significance. Singing refers to 0p- Stabihty greater than it would be Without -f eration such that an impressed small disturbance back. That type of amplifier is disclosed for exwhich itself dies out results in a response that ample the above mentioned Qopehdihg applifi does not die out, b t goes on indefinitely either cations and in the above mentioned British pat staying at a relatively small value 5r increasing and in British Patents 3. and & 2.

until it is limited by the non-linearity of the 'sys- In Such amplifiers" Where tube modulation tem. The amplification of a vacuum tube ampliductioh 0 modulation components of given fier With-cut feedback is designated ,l and is the quencies iseto be large, it is proportional to the complex quantity by which the voltage on the gain, (for those modulation COmPOHBIItS) in a grid of thefirst tube must be multiplied by to ob- Singletrip around e 010m feed-back tain the phase and magnitudeof the total resultconsequently that gain Should be The ing voltage in the plate circuit of the last tube. modulation components that it is desired tore- Amplification ratio is the absolute or scaler value due? feedback are usually Waves of of a. Gain is twenty times the logarithm of the quehcies Within the utilized frequency range an'lplifica tion The fi represents e. g. the range Of-;the frequencies Of the the propagationonce around the closed feedback signal F to be amphfied t t amplifierloop of a feedback amplifier. ltfollows that p pracfilce, when t 13 the designates the complex quantity by which a drivlbel galh for t Smgle trlp around the 1001)) m voyage in the space path of'th last tube, in is large'for the frequencies of the utilized freseries with the plate-filament impedance in that qhehcy range it is greater than Zero for some tube, must be multiplied to give the'voltage that hlgher frequency and for some lower frequency itthe driving voltage alone-acting through the and t 1001) Phase shift P- the phase Shift feedback path, will produce on the grid of the expenehcefi Waves lh'pftsslhg once aroundfirst tube. As shown in a copending application the loop) 15 e or multlple of for any of H. s. Black, sci-n1 No; 606,871, filed April 22, frequency at Winch the loop a equals 1932 for Wave translation systems, the amplifiseeds Zero decibels the'a'mphfiel may Sing at cation of a feedback m lifie is that frequency. (As indicated in the above mentioned :copending application, Serial No. .JL. 1 a 606,871; a criterion for-freedom from singing is j 1- given by Nyquists rule, in his article. on Re: 45 and the corresponding change in amplification generation theory, B611 System Technical f caused by the feedback action is 11311, J y, 1932, P s 6 o 1471 Such criterion is given also in the above mentioned Brit- L. ish Patent 374,130.) Moreover, passive networks introduced in the loop to contribute a compo- 5Q The quantity 7 nent of loop attenuation increasing with fre- 1 quencyordinarily introduce a component of loop m 7 phase shift that tends to lower the frequency,

and raise the-gain, at which the loop phaseshift is a quantitative measure of the amount of fecdireaches a given multiple. of 3609-. To avoid th 4'15 singing condition, it is desirable to control the loop phase shift and the loop gain carefully with respectto the entire frequency spectrum. (Such control is desirable also for other reasons, as for example-to prevent loop phase shift from causing increase in thegain of the amplifier, which increase may be undesirable because ac companied by a corresponding increase in the modulation products. This increase in gain may become a limiting factor determining the per missible loop phase shift at high frequencies where it becomes difiicult to obtain sufficient' feedback. At other frequencies it is usually not a limiting factor. For example, assuming the amplifier does not sing, the amplifier gain change produced by feedback will be within 1 decibel of the gain around the feedback loop regardless of phase shift if the gain around'the loop exceeds 20 decibels. Further, the feedback does not increase the amplifier gain if the loop gain exceeds 6 decibels.) If the value of the loop phase shift were maintained at i180 it would be as remote as possible from the potential singing values of 0 and multiples of 360;however, in practice it is not necessary to attain this condition. The requirement for freedom from singing will always be met if for every frequency of loop gain, (1. e., every frequency at which the loop gain is zero or greater) the loop phase shift differs from zero and every multiple of 360", or in other words if the loop phase shift frequency characteristic does not cross nor touch the zero phase shift axis in the frequency range of loop gain. (It is not to be inferred that this requirement is always essential for freedom from singing. A criterion for such freedom is given by Nyquists rule, as referred to above.)

In designing an amplifier with negative feedback for distortion reduction, assuming the vacuum tube or tubes of each stage in the loop to introduce a phase shift having a-constant component of 180. (in addition to any component due to interelectrode capacitance; for example), the number of vacuum tube stages used in the loop may be made either odd or even, to facilitate control of singing tendency; The question whether an odd or an even. number is .more suitable will depend upon whether the loop is made to have phase reversing means other than the tubes, and upon what other phase shifts are present in the loop. If, over the frequency range of loop gain the constant component of the total loop phase shift is any odd. multiple of 180, then it is necessary that the total vari-.

ation of theloop phase shift with frequency over that frequency range be maintained within limits of +180 and 180 in order to make the total loop phase shift differ from zero and every multiple of 360 for every frequency in that frequency range.

The difficulty of insuring that the variation of loop phase shift with frequency is maintained within the required limits over the frequency range of loop' gain is in general increased'by the fact that, (as brought out, for exam'plain the above mentioned copending application), when the distortion reduction and associated amplifier gain reduction produced by feedback action is to be large, the gain ofthe amplifier without feedback must then correspondingly exceed the gainrequired with feedback; because when the gainwithout feedback,requiredto produce the desired amount of distortion Ta ducing feedback and the desired amount of gain 'with feedback, necessitatesuse of a plurality of stages and a plurality of interstage coupling circuits, the phase shifts around the 7 closed loop may become large at low frequencies 7 or high frequencies, or both, especially at frequencies well below or well above theutilized 7 range. For example, the phase shifts may .become large at low frequencies because of inter stage coupling condensers, and at high frequen ciesbecause of shunt capacitance, for instance, tube and wiring capacities. The singing tendency may become particularly troublesome when the amplifier. is called upon ,to transmit wide quencies, for example. 7 c V In its specific aspect mentioned above, the invention isa negative feedback amplifier'ineluding a screen grid tube having in its screen.

grid lead a frequency selective impedance that causes the screen grid .to assume potentials which produce in the amplifier, as indicated hereinafter, phase shift and gain reduction that frequency bands extending to very high frereduce the singing tendency without undulyaf V fecting operation of the. amplifier. deleteriously in the utilized frequency range. V

As indicated, above, loop gain and loop phase shift present important limitations. in operation of feedback amplifiers, especially the loopgain Y and loop phase shift at low and high frequencies in operation of wide band negative feedback amplifiers for reducing modulation or distortion by feedback action.

An object of the invention is to so control the loop gain and loop. phase shifts as to reduce, singing tendencyor to increase the distortion suppression obtainable in such amplifiers, or both.

Other objects and aspects of the: invention will be apparent from the following description and claims.

Inthe accompanying drawings, Fig.1 is a circuit diagram of an amplifier embodying'the'speciflc aspect of the invention referred to above, with impedances in its screen grid circuits shownas generalized impedances;

Fig. 2 is a circuitdiagram facilitating explana-" tion of the-action of the impedances;

Figs. 3, 4, 5 and 6 show examples, of specific forms the impedances may have; and,

Figs. 3A and 3B arevector diagrams facilitate ing explanation of the action of the impedances shown in Figs. 3, 4, 5 and 6.

In Fig. 1 a network amplifier amplifies waves f received by input transformer T from line" or circuit L and transmits the amplified waves through output transformer T to line or. circuit L. The amplifier comprises. a forwardly'transmitting path shown by way of example as-including three impedance coupled screen gridvac uum tubes 1, 2 and 3 in tandem, and comprises a feedback path F which may, if desired, include a network N for transmission control.

The amplifier may be of the general type' refor operation Without feedback, in order to reduce distortion correspondingly and render the gain stability greater than for operation without feedback.

The network :N maybe, for instance, an at tenuation equalizer, with its 'rattenuation-frequency characteristic like that of ithe cableor line in which the amplifier is connected; for .then,

as explainedin the above mentionedcopending application 606,871 or British Patent 371,887, the amplifier will equalize the line attenuation.

By Way of example, tubes 1 and 2 are shown as of the heater type,. a battery A is shown as supplying filamentheating current forall of the tubes, a battery B is shown as supplying space current for all of the tubes, a battery5 is shown Each of the two interstage coupling circuits includes a plate retard orchoke coil 10, a stopping condenser 11 and a grid leak resistor 12. 1 Impedance may be connectedin the screen circuit of one or more of the tubes to controlthe phase or amplitude, or both, of waves, transmitted by the tube. For instance, each tube is shown as having impedances-Z1, Z2, and Z3 in its screen circuit, and these impedances can be used for'frequency selective phase or amplitude control, for example, to reduce singing tendency of the amplifier.

Fig. 2 shows a screen grid vacuum tube circuit such as thatof each stage of Fig. 1. When each of the three impedances Z1, Z2, and Z3 is zero, the tube circuit is a-normal screen grid tube circuit and the voltage V; across the load impedance Z is the same as if the tube were a three element tube, and so is given by the equation:

V--p. e o n:

the symbols e and R and also the symbols 1, Rs, E and c2 which are referred to below, having the significance indicated in Fig. 2, t, n and 2 representing the' tube amplification constants, respectively from control grid to plate circuit, control grid to screen grid circuit, and screen gridto plate circuit. When each of the two impedances Z2 and Z3 is zero, and Zr is not zero,

ll #1 8+ Z81 and therefore vFor ideal screening action, (in which #:mm

and RS=RO), Equation (1) becomes:

the leie menin es? that ex r sses the-change in amplification ratiointroduced b the action of the screen grid.

If R is 'not equal toRs and is not equal to paEquation (1) is'used in determining V.

It has-beenfound by measurements that ,u' is very nearly equalto [Ll/L2, but Rs is much smaller than Rs, (about one-tenth of the value of R0). This is advantageous, in that the smaller the value of Rs .the greater is the percentage of me across Z1 and therefore the more effec-' tively does the screen grid control the amplification of the circuit. v r

When none. oftheimpedances Z1, Z2 and Z3 is restricted to zero value, instead of Equation (1), we have s+ l)( 2+ 3)+ 2 3J o+ o In feedback amplifiers of the general type shown in Fig. 1, the loop phase shift introduced by the elements other than the tubes commemly changes with frequency from large negative values at frequencies far belowthe utilized frequency range, to large positive values at frequencies far above the utilized range For example, the phase shift may be negative for low frequencies in the transmitting range and below, and positive for high frequencies in the transmitting range and above. As indicated above, this phase shift may produce objectionable effects, notably singing around the loop. Compensation for this phase shift and avoidance of the singing or other objectionable effects, can be accomplished by connecting impedance in the screen circuit of one or more'of the vacuum tube stages. V

Forexample, Fig; 3 shows an inductance coil 20 connected as impedance Z1 in the screen lead of a tube. inductance in the screen lead in troduces positive phase'shift in waves trans mitted through (amplified by) the tube, and capacity in the screen lead introduces negative phaseshift in these waves. Therefore, the inductance of coil 20 can be made to compensate for deleterious negative phase shift at the low frequencies; and moreover, the coil' 20 can be made to compensate for deleterious positive phase shift atthe high frequencies, by virtue of its capacitive. reactance above its resonance frequency, the coil being designed to have its resonance frequency below the frequencies at which it is to compensatev for the positive phaseshift. Any phase shifts introduced by the coil at fre-' quencies other than the frequencies at which the coil reduces deleterious phase shift, need'not be objectionable. For example, they can be made to occur in a frequency region where they are tolerable from the standpoint of avoiding singing around the feedback loop. The feedback action of the amplifier reduces any variation of the amplifier gain with frequency introduced by the coil 20in the utilized frequency band, where 43 for the feedback amplifier is large compared to'unity. vThe inherent capacity of coil 20 is indicated at,29. If desired it can be augmented by additional capacity in the form of aseparate condenser.

Fig. 3A shows the relationbetween ,u/c and l ,uzE fol the low; frequencies, for whichthe coil 20 acts as. an inductance.

for the high frequencies, for which the "coil .20 acts as a capacity. V

In Fig. 3A the addition. of 42E :to We has resulted in a voltage R which lags we by angle 0; and in Fig. 3B the addition of {112E to 'e has resulted in a voltage R. which :leads angle "0.

Feedback amplifiers -of the general type shown in Fig. 1 sometimes tend to singat low frequencies in the utilized frequency range, or at lower frequencies, for example frequencies farabelow the utilized range. For instance in the case of a 4 kilocycle to '40 :kilocycle amplifier, singing tendency may be troublesome in the lower-part of the operating frequency band or ata frequency such as 200 cycles, or at so-called thump frequencies, say below 10 cycles per second, or may quencies, the amplification ratio and the gain being reduced by the action of Z1 as indicated by the fact that R is shown shorter than i'e. The phase shift introduced by the action Z1 is negative, which likely to be of the wrong sign for utilization to compensate for deleterious phase shift in the loop; but the phase shift introduced by 21 can be made to have its maximum at a frequency above the low frequencies at which the singing tendency is troublesome and below at the latter frequencies whereas the gain reduction introduced by the impedance Z1 can at the same time be made large over the frequency range of the latter frequencies and small for higher .frequencies. With decreasing frequency, as the reactance of the capacity 30'becomes very high so that the effect of the capacity is substantially negligible, resistance 31 is highly elfective in reducing gain. Therefore the capacityv and resistance can be used to reduce the loop gain of aifeedback amplifier belo wfzerobefore increase of loop phase shift with frequency decrease causes the loop phase shift to "reach -a potential singing value of zero or a multiple of 360 degrees. i

Fig. 5 shows impedance Z1 combining "the impedances Z1 of Figs. 3 and 4, 'too'btain'the advantages-of each 'in the same tube "circuit; The coil 20 can be made effectively a short circuit or 7 of zero impedance to low frequencies for which resistance 31 is used to reduce gain; and the cond'enserB'O can be made to short "circuit resistance 31 for frequencies at and below those for whichcoil 2O is'used to control phase shift.

Anyof the circuits'of Figs. 3, l'or 5 can be employed in any of the stages of an amplifier such as that-of Fig.1; For'example, the circuits of Figs-B, *4 and '5 maybe used in stages 1, 2 and 3, respectively of Fig. 1. Various forms of impedancesmay be used for the-elements Z1, Z2 and Z3 in Fig. 1. Examples havebeengiven of forms that impedances Z1 can have; and Fig. fi 'shows, by way of example, forms which impedanc'e's Z2 and 5Z3 may take. In Fig. 6, Z3 is a. resistance and Z2 is a condenser. Thesetwo elements may be for instance 'a. noise filter, the capacity 'of Z:

ii-poems 7 being sufficient :to "make the waluerof Z; 'substailtially-zero :or negligiblysmall.= What is claimed is: l. A wave translating system 'conip ifl il Bivacuum tube having 'a'control grid and an auxiliary grid and'auxiliary grid circuit, a cap'acitativeload circuit for said tube, the capacitive character 0!.

said "load circuit causing the component ofithe. 1'

voltage across said load circuit that isdue only to the voltageon the control grid to be retarded in phase with respect to the control grid voltage, and means "comprising impedance in said am:-

iliaryigrid circuit adapted to produce on said .7

auxiliary :grid a voltage advancing the phase "of Q the voltage :across said load circuit withiirespec't to the :co'ntrol grid voltage and thereby zcom V pensating for the retardation of theload'clrcuit.

voltage, with'respect .-t0 the control grid voltage, caused by the capacitive character of said load circuit. l

2. n 'wave 'amplifymg system compnsingian amplifier including a vacuum tube diaving I screen grid, means forming with said amplifier a closed feedback loop producing negative steed back therein for rendering modulation therein less than 'without feedbackyandmeansiproducing.

potentials :on' said grid that-reduce tendency of the system to sing aroundsaid loop.

3. A Wave amplifying system. comprising an 7 amplifier including a vacuum tube having a screen grid'and a screen grid leadpmeans fornfin'g withsaid amplifier a closed feedbackloopproduring negative feedback therein :in' the utilized.

frequency band; a source of unidirectional volt ageinsai'd lead for producing positivepotential on said screen grid, and impedance, :coinprising reactance, connected in series with said source g in said lead and adapted tocause voltage drop across said impedance resulting from current flow in said impedance to produce screen grid. potentials reducing tendency of the system -to.

sing around said 'loop' outside theutilizedifre quencyrange.

4.'A wave amplifying system comprising an amplifier including a vacuum tube having a screen grid and a screen grid lead, means forming with said amplifier-a closed feedback lo'bp producing' ne'gative feedback therein fora given 7 frequency range, and frequency selective means in said lead adapted to produce in said loop-a component of loop phase shift so related to the remaining loop phase shift as to reduce tendency of the system to sing around saidloopfioutside said frequency range. v 4

IS. A wave amplifying system comprising an amplifier includinga vacuum 'tubehaving a cathode, a screen grid and a screen grid lea'd,:means forming with said' :amplifier'ia closed loop producing negative feedback therein for a.

1 given frequency range,-and a source of unidi-' rectional'voltageandfrequency selective means in serial relation in said lead for reducing the-gain of said amplifier at frequencies outside said fre quency range.

6. A wave amplifying system comprising amplifier including a vacuum tube having a screen grid and a screen grid "lead, means forming with said amplifier ahclosed feedback loop producing negative feedback therein for a given frequency range, and a source of unidirectional voltage-and frequency selective means in serial relation in. said lead for producing gainreduction andfphase 'shift in saidamplifier that re; duce'tendency of the system to singaro'und said loop-at frequencies outsldesaid rrequencymige. if

'7. A wave translating system comprising an to reduce tendency of the system to sing around amplifier including a vacuum tube having a said 100p, said impedance comprising areactance screen grid and a screen grid lead, means formincluding an inductance coil and a condenser ing with said amplifier a closed feedback loop in serial relation therein and a resistor connected 5 producing negative feedback therein in the utiacross a portion of said reactance.

lized. frequency band, and impedance in said lead ALPHONS V. WURMSER. 

